Audio amplifiers amplify AC signals in the frequency range of approximately 20 to 20,000 hertz. They may amplify the whole audio range or only a small portion of it. Audio amplifiers are divided into two categories: voltage amplifiers and power amplifiers.
Voltage amplifiers are primarily used to produce a high voltage gain. Power amplifiers are used to deliver a large amount of power to a load. For example, a voltage amplifier is typically used to increase the voltage level of a signal sufficiently to drive a power amplifier. The power amplifier then supplies a high output to drive a load such as a loudspeaker or other high-power device.
Typically, voltage amplifiers are biased to operate as class A amplifiers, and power amplifiers are biased to operate as class B amplifiers.
Audio Amplifier Operation
Voltage Amplifiers
Figure 1 shows a simple voltage amplifier. The circuit shown is a common-emitter circuit. It is biased class A to provide a minimum amount of distortion.
The amplifier can provide a substantial voltage gain over a wide frequency range. Because of the coupling capacitors, the circuit cannot amplify a DC signal.
Two or more voltage amplifiers can be connected together to provide higher amplification. The stages may be RC coupled or transformer coupled. Transformer coupling is more efficient. The transformer is used to match the input and output impedance of the two stages.
This keeps the second stage from loading down the first stage. Loading down is the condition when a device creates too large a load and severely affects the output by drawing too much current.
The transformer used to link the two stages together is called an inter-stage transformer. Once a sufficient voltage level is available, a power amplifier is used to drive the load.
Power Amplifiers
Power amplifiers are designed to drive specific loads and are rated in watts. Typically a load may vary from 4 to 16 ohms.
Figure 2 shows a two-transistor power amplifier circuit, called a push-pull amplifier. The top half of the circuit is a mirror image of the bottom half. Each half is a single transistor amplifier. The output voltage is developed across the primary of the transformer during alternate half-cycles of the input signal. Both transistors are biased either class AB or class B.
The input to a push-pull amplifier requires complementary signals. That is, one signal must be inverted compared to the other. However, both signals must have the same amplitude and the same frequency. The circuit that produces the complementary signal is called a phase splitter.
A single-transistor phase splitter is shown in Figure 3. The complementary outputs are taken from the collector and the emitter of the transistor. The phase splitter is operated as a class A amplifier to provide minimum distortion. The coupling capacitors are necessary to offset the differences between the DC collector and emitter voltages.
A push-pull amplifier that does not require a phase splitter is called a complementary push-pull amplifier. It uses an NPN and a PNP transistor to accomplish the push-pull action (Figure 4). The two transistors are connected in series with their emitters together.
When each transistor is properly biased, there is 0.7 volt between the base and emitter or 1.4 volts between the two bases. The two diodes help to keep the 1.4-volt difference constant. The output is taken from between the two emitters through a coupling capacitor.
For amplifiers greater than 10 watts, it is difficult and expensive to match NPN and PNP transistors to ensure that they have the same characteristics.
Figure 5 shows a circuit that uses two NPN transistors for the output-power transistors. The power transistors are driven by lower-power NPN and PNP transistors while the upper set of transistors is connected in a darlington configuration. The lower set of transistors uses a PNP and an NPN transistor. Operating as a single unit, they respond like a PNP transistor.
This type of amplifier is called a quasi-complementary amplifier. It operates like a complementary amplifier but does not require high-power complementary output transistors.
Because of the large amounts of power generated by power amplifiers, some components get hot.
Video Amplifier Operation
Video amplifiers are wideband amplifiers used to amplify video (picture) information. The frequency range of the video amplifier is greater than that of the audio amplifier, extending from a few hertz to 5 or 6 megahertz. For example, a television requires a uniform bandwidth of 60 hertz to 4 megahertz. Radar requires a bandwidth of 30 hertz to 2 megahertz.
In circuits that use saw-tooth or pulse waveforms, it is necessary to cover a range of frequencies from one-tenth of the lowest frequency to ten times the highest frequency. The extended range is necessary because non-sinusoidal waveforms contain many harmonics and they must all be amplified equally.
Because video amplifiers require good uniformity in frequency response, only direct or RC coupling is used. Direct coupling provides the best frequency response, whereas RC coupling has economic advantages.
The RC-coupled amplifier also has a flat response in the middle frequency range that is suitable for video amplifiers. Flat response is the term used to indicate that the gain of an amplifier varies only slightly within a stated frequency range. The response curve plotted for such an amplifier is almost a straight line; hence the term “flat response.”
A factor that limits the high-frequency response in a transistor amplifier is the shunt capacitance of the circuit. A small capacitance exists between the junctions of the transistor. The capacitance is determined by the size of the junction and the spacing between the transistor’s leads. The capacitance is further affected by the junction bias. A forward-biased base-emitter junction has a greater capacitance than a reverse-biased collector-base junction.
To reduce the effects of shunt capacitance and increase the frequency response in transistor video amplifiers, peaking coils are used. Figure 6 shows the shunt-peaking method. A small inductor is placed in series with the load resistor.
At the low- and mid-frequency range, the peaking coil will have little effect on the amplifier response. At the higher frequencies, the inductor resonates with the circuit’s capacitance, which results in an increase in the output impedance and boosts the gain.
Another method is to insert a small inductor in series with the interstage coupling capacitor. This method is called series peaking (Figure 7). The peaking coil effectively isolates the input and output capacitance of the two stages.
Often series and shunt peaking are combined to gain the advantages of both (Figure 8). This combination can extend the bandwidth to over 5 megahertz.
The most common use of video amplifiers is in television receivers and computer monitors. (Figure 9).
Transistor Q1 is connected as an emitter-follower. Input to transistor Q1 is from the video detector. The video detector recovers the video signal from the intermediate frequency.
In the collector circuit of transistor Q2 is a shunt-peaking coil (L1). In the signal-output path is a series-peaking coil (L2). The video signal is then coupled to the picture tube through coupling capacitor C5.
Radio Frequency | RF Amplifier Operation
RF (radio-frequency) amplifiers usually are the first stage in an AM, FM, or TV receivers and are similar to other amplifiers. They differ primarily in the frequency spectrum over which they operate, which is 10,000 to 30,000 megahertz. There are two classes of RF amplifiers: untuned and tuned.
In an untuned amplifier, a response is desired over a large RF range, and the main function is amplification. In a tuned amplifier, high amplification is desired over a small range of frequencies or a single frequency. Normally, when RF amplifiers are mentioned, they are assumed to be tuned unless otherwise specified.
In receiving equipment, the RF amplifier serves to amplify the signal and select the proper frequency. In transmitters, the RF amplifier serves to amplify a single frequency for application to the antenna. Basically, the receiver RF amplifier is a voltage amplifier, and the transmitter RF amplifier is a power amplifier.
In a receiver circuit, the RF amplifier must provide sufficient gain, produce low internal noise, provide good selectivity, and respond well to the selected frequencies.
Figure 10 shows an RF amplifier used for an AM radio. Capacitors C1 and C4 tune the antenna and the output transformer T1 to the same frequency. The input signal is magnetically coupled to the base of transistor Q1. Transistor Q1 operates as a class A amplifier. Capacitor C4 and transformer T1 provide a high voltage gain at the resonant frequency for the collector load circuit. Transformer T1 is tapped to provide a good impedance match for the transistor.
Figure 11 shows an RF amplifier used in a television VHF tuner. The circuit is tuned by coils L1A, L1B, and L1C. When the channel selector is turned, a new set of coils is switched into the circuit. This provides the necessary bandwidth response for each channel. The input signal is developed across the tuned circuit consisting of L1A, C1, and C2.
Transistor Q1 operates as a class A amplifier. The collector-output circuit is a double-tuned transformer. Coil L1B is tuned by capacitor C4, and coil L1C is tuned by capacitor C7. Resistor R2 and capacitor C6 form a decoupling filter to prevent any RF from entering the power supply to interact with other circuits.
In an AM radio, the incoming RF signal is converted to a constant IF (intermediate frequency) signal. A fixed-tuned IF amplifier is then used to increase the signal to a usable level.
The IF amplifier is a single-frequency amplifier. Typically, two or more IF amplifiers are used to increase the signal to the proper level. The sensitivity of a receiver is determined by its signal-to-noise (S/N) ratio. The higher the gain, the better the sensitivity.
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